1. Field of Invention
The present invention relates to an active bleeder circuit capable of triggering a tri-electrode AC switch (TRIAC) in all phase, and a light emitting device power supply circuit and a TRIAC control method using the active bleeder circuit. Particularly, it relates to an active bleeder circuit which is capable of triggering a TRIAC circuit in all phase, but does not consume power when a TRIAC circuit is not provided, and a light emitting device power supply circuit and a TRIAC control method using the active bleeder circuit.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art light emitting diode (LED) power supply circuit 100. As shown in FIG. 1A, the LED power supply circuit 100 includes a tri-electrode AC switch (TRIAC) dimmer circuit 12, a rectifier circuit 14, and an LED driver circuit 16. The TRIAC dimmer circuit 12 receives an AC input signal at an AC input node VL, and generates a phase-cut AC dimming signal at an phase-cut AC dimming node VL′. When the AC input signal exceeds a trigger phase, a TRIAC device in the TRIAC dimmer circuit 12 is triggered to turn ON. FIG. 1B shows the waveform of the AC input signal VL (dashed line) received by the TRIAC dimmer circuit 12 and the waveform of the phase-cut AC dimming signal VL′ (solid line) generated by the TRIAC dimmer circuit 12. The rectifier circuit 14 receives the phase-cut AC dimming signal VL′ and a neutral signal at a neutral node VN, and rectifies the voltage difference between them, to generate a rectified dimming signal at a rectified node VD. The rectified dimming signal VD is inputted to the LED driver circuit 16 to drive an LED circuit 11. In the above circuit, the TRIAC dimmer circuit 12 is provided for adjusting an average brightness of the LED circuit 11 by tuning the trigger phase of the phase-cut AC dimming signal VL′.
One drawback of the aforementioned prior art is that the TRIAC dimmer circuit 12 includes a TRIAC device which needs a relatively high latching current to trigger. If the load circuit driven by the power supply circuit 100 is a circuit consuming high power such as an incandescent bulb, the high latching current is naturally generated and therefore is not a considerable issue. However, when the power supply circuit 100 is driving a light load circuit such as the LED circuit which consumes low power, the high latching current requirement becomes a considerable issue. If the latching current is not high enough to trigger the TRIAC device, the TRIAC device misfires and the LED circuit 11 flickers. FIG. 1C shows the waveform of the phase-cut AC dimming signal VL′ when the TRIAC misfires, which causes the LED circuit 11 to flicker.
FIGS. 2A and 2B show schematic diagrams of another prior art LED power supply circuit 110, which is proposed to mitigate the aforementioned LED flicker issue. As shown in FIG. 2A, the prior art LED power supply circuit 110 includes a bleeder circuit 18 in additional to the TRIAC dimmer circuit 12, the rectifier circuit 14, and the LED driver circuit 16. The bleeder circuit 18 is coupled between the rectifier circuit 14 and the LED driver circuit 16, for generating a sufficient latching current periodically to trigger the TRIAC device in the TRIAC dimmer circuit 12. After the TRIAC device is triggered, the latching current generated by the bleeder circuit 18 is consumed by a loop connected to ground. FIG. 2B shows a specific embodiment of the LED power supply circuit 110 including the bleeder circuit 18.
More specifically, the bleeder circuit 18 includes resistors R1 and R2, which are connected in series between two output nodes of the rectifier circuit 14. A divided voltage across the resistor R2 turns ON a switch Q1, which generates the latching current for the TRIAC device. A resistor R3 and Zener diodes ZD1 and ZD2 are connected in series; after the switch Q1 is turned ON, a divided voltage at the node between the resistor R3 and the Zener diode ZD1 turns ON the switch Q2, such that a holding current is generated and flows through a resistor R4. The waveforms of the AC input signal VL, the phase-cut AC dimming signal VL′, and a TRIAC current IT are shown in FIG. 2C.
Even though the prior art LED power supply circuit 110 shown in FIGS. 2A and 2B mitigates the LED flicker issue caused by the misfire of the TRIAC device, this prior art has a drawback that the TRIAC device in the TRIAC dimmer circuit 12 can not be triggered in all phase. More specifically, for the bleeder circuit 18 to generate the latching current to trigger the TRIAC device, the rectified dimming signal generated by the rectifier circuit 14 at the rectified node VD must be higher than a certain level such that the divided voltage across the resistor R2 is higher than the threshold voltage of the switch Q1. If a user intends to turn low the brightness of the LED circuit 11 to an extent that the rectified dimming signal is too low, i.e., if the ON phase of the rectified dimming signal in FIG. 1B or 2C is too short such that the trigger phase is too close to the end of the period of the phase-cut AC dimming signal (referring to FIG. 4A), the divided voltage across the resistor R2 will be lower than the threshold voltage of the switch Q1, and the TRIAC device can not be triggered because no latching current is generated. In other words, in this prior art which uses the bleeder circuit 18, a user can not use the TRIAC dimmer circuit 12 to adjust the brightness of the LED circuit 11 in full range (the TRIAC device in the TRIAC dimmer circuit 12 can not be triggered in all phase), and there is a limit to the latest timing of the trigger phase. Furthermore, in certain applications it is not necessary to provide the dimming function and therefore the TRIAC dimmer circuit 12 is not required, but in this prior art, even though there is no TRIAC dimmer circuit 12, the bleeder circuit 18 still generates current and consumes power which is completely wasted.
In view of the foregoing, the present invention provides an active bleeder circuit, and a light emitting device power supply circuit and a TRIAC control method using the active bleeder circuit, wherein the active bleeder circuit is capable of triggering a TRIAC circuit in all phase, but does not consume power when a TRIAC circuit is not provided.